diff --git a/docs/functions/other.rst b/docs/functions/other.rst
index 763091ec..96097428 100644
--- a/docs/functions/other.rst
+++ b/docs/functions/other.rst
@@ -134,3 +134,54 @@ Other
 
    Constructs a :js:func:`KDE` distribution from a sample based
    marginal distribution.
+
+.. js:function:: AIS(model[, options])
+
+   Returns an estimate of the log of the normalization constant of
+   ``model``. This is not an unbiased estimator, rather it is a
+   stochastic lower bound. [grosse16]_
+
+   The sequence of intermediate distributions used by AIS is obtained
+   by scaling the contribution to the overall score made by the
+   ``factor`` statements in ``model``.
+
+   When a model includes hard factors (e.g. ``factor(-Infinity)``,
+   ``condition(bool)``) this approach does not produce an estimate of
+   the expected quantity. Hence, to avoid confusion, an error is
+   generated by ``AIS`` if a hard factor is encountered in the model.
+
+   The length of the sequence of distributions is given by the
+   ``steps`` option. At step ``k`` the score given by each ``factor``
+   is scaled by ``k / steps``.
+
+
+   The MCMC transition operator used is based on the :ref:`MH kernel
+   <mh>`.
+
+
+   The following options are supported:
+
+   .. describe:: steps
+
+      The length of the sequence of intermediate distributions.
+
+      Default: ``20``
+
+   .. describe:: samples
+
+      The number of times the AIS procedure is repeated. ``AIS``
+      returns the average of the log of the estimates produced by the
+      individual runs.
+
+      Default: ``1``
+
+   Example usage::
+
+     AIS(model, {samples: 100, steps: 100})
+
+.. rubric:: Bibliography
+
+.. [grosse16] Grosse, Roger B., Siddharth Ancha, and Daniel M. Roy.
+              "Measuring the reliability of MCMC inference with
+              bidirectional Monte Carlo." Advances in Neural
+              Information Processing Systems. 2016.
diff --git a/docs/inference/methods.rst b/docs/inference/methods.rst
index ea4ae2ac..ec0b0d97 100644
--- a/docs/inference/methods.rst
+++ b/docs/inference/methods.rst
@@ -125,6 +125,8 @@ Kernels
 
 The following kernels are available:
 
+.. _mh:
+
 .. describe:: MH
 
    Implements single site Metropolis-Hastings. [wingate11]_
diff --git a/src/header.js b/src/header.js
index 0d1e112c..b174fcc0 100644
--- a/src/header.js
+++ b/src/header.js
@@ -32,6 +32,7 @@ try {
   var smc = require('./inference/smc');
   var rejection = require('./inference/rejection');
   var incrementalmh = require('./inference/incrementalmh');
+  var ais = require('./inference/ais');
   var optimize = require('./inference/optimize');
   var forwardSample = require('./inference/forwardSample');
   var checkSampleAfterFactor = require('./inference/checkSampleAfterFactor');
@@ -186,7 +187,7 @@ module.exports = function(env) {
   // Inference functions and header utils
   var headerModules = [
     enumerate, asyncpf, mcmc, incrementalmh, pmcmc,
-    smc, rejection, optimize, forwardSample, dreamSample, checkSampleAfterFactor,
+    smc, rejection, optimize, ais, forwardSample, dreamSample, checkSampleAfterFactor,
     headerUtils, params
   ];
   headerModules.forEach(function(mod) {
diff --git a/src/header.wppl b/src/header.wppl
index 24211ae8..dd118488 100644
--- a/src/header.wppl
+++ b/src/header.wppl
@@ -473,6 +473,15 @@ var OptimizeThenSample = function(wpplFn, options) {
   return SampleGuide(wpplFn, opts);
 };
 
+// Make AIS available via Infer to allow AIS to be tested using
+// existing inference tests.
+var AISforInfer = function(wpplFn, options) {
+  var dummyMarginal = Infer(constF(true));
+  // This mutates dummyMarginal.
+  _.assign(dummyMarginal, {normalizationConstant: AIS(wpplFn, options)});
+  return dummyMarginal;
+};
+
 /*
  * DefaultInfer() called when no options are specified
  * var maxEnumTreeSize: upper bound for enumeration tree size, enumeration ends when above threshold.
@@ -538,6 +547,7 @@ var Infer = function(options, maybeFn) {
     incrementalMH: IncrementalMH,
     forward: ForwardSample,
     optimize: OptimizeThenSample,
+    AIS: AISforInfer,
     defaultInfer: DefaultInfer
   };
 
diff --git a/src/inference/ais.js b/src/inference/ais.js
new file mode 100644
index 00000000..a599e025
--- /dev/null
+++ b/src/inference/ais.js
@@ -0,0 +1,76 @@
+// This closely follows the AIS implementation developed for WebPPL as
+// part of "Measuring the reliability of MCMC inference with
+// bidirectional Monte Carlo" (Grosse et al).
+
+// https://arxiv.org/abs/1606.02275
+// https://github.com/siddancha/webppl/tree/b607efe714d78c44f763ffd36324c0b67de96f56
+
+'use strict';
+
+var _ = require('lodash');
+var assert = require('assert');
+var util = require('../util');
+var numeric = require('../math/numeric');
+
+module.exports = function(env){
+
+  var Initialize = require('./initialize')(env);
+  var kernels = require('./kernels')(env);
+
+  function AIS(s, k, a, wpplFn, options) {
+    options = util.mergeDefaults(options, {
+      steps: 20,
+      samples: 1
+    });
+
+    var weights = [];
+
+    var singleSample = function(k) {
+
+      var initialize, run, finish;
+
+      initialize = function() {
+        return Initialize(run, wpplFn, s, env.exit, a, {});
+      };
+
+      run = function(initialTrace) {
+
+        var curStep = 0;
+        var increment = 1 / options.steps;
+        var weight = 0;
+
+        var MHKernel = kernels.parseOptions('MH');
+
+        var mhStepKernel = function(k, trace) {
+          weight += increment * trace.scoreAllFactors();
+          curStep += 1;
+          return MHKernel(k, trace, {
+            factorCoeff: curStep * increment,
+            allowHardFactors: false
+          });
+        };
+
+        var mhChainKernel = kernels.repeat(options.steps, mhStepKernel);
+
+        return mhChainKernel(function(trace) {
+          return k(weight);
+        }, initialTrace);
+      };
+
+      return initialize();
+    };
+
+    return util.cpsLoop(options.samples, function(i, next) {
+      return singleSample(function(weight) {
+        weights.push(weight);
+        return next();
+      });
+    }, function() {
+      var avgWeight = numeric._sum(weights) / options.samples;
+      return k(s, avgWeight);
+    });
+  }
+
+  return {AIS: AIS};
+
+};
diff --git a/src/inference/mhkernel.js b/src/inference/mhkernel.js
index 7a4f36d1..9ad8e06e 100644
--- a/src/inference/mhkernel.js
+++ b/src/inference/mhkernel.js
@@ -30,12 +30,18 @@ module.exports = function(env) {
 
     runOpts = util.mergeDefaults(runOpts, {
       proposalBoundary: 0,
-      exitFactor: 0
+      exitFactor: 0,
+      factorCoeff: 1,
+      allowHardFactors: true
     });
 
     this.proposalBoundary = runOpts.proposalBoundary;
     this.exitFactor = runOpts.exitFactor;
 
+    this.factorCoeff = runOpts.factorCoeff;
+    assert.ok(0 <= this.factorCoeff && this.factorCoeff <= 1);
+    this.allowHardFactors = runOpts.allowHardFactors;
+
     this.cont = cont;
     this.oldTrace = oldTrace;
     this.a = oldTrace.baseAddress; // Support relative addressing.
@@ -61,6 +67,9 @@ module.exports = function(env) {
   MHKernel.prototype.factor = function(s, k, a, score) {
     // Optimization: Bail early if we know acceptProb will be zero.
     if (ad.value(score) === -Infinity) {
+      if (!this.allowHardFactors) {
+        throw new Error('Hard factor statements are not allowed.');
+      }
       return this.finish(this.oldTrace, false);
     }
     this.trace.numFactors += 1;
@@ -203,10 +212,20 @@ module.exports = function(env) {
     // assert(_.isNumber(ad.value(oldTrace.score)));
     // assert(_.isNumber(this.regenFrom));
     // assert(_.isNumber(this.proposalBoundary));
-
     var fw = this.transitionProb(oldTrace, trace, this.fwdProposalDist);
     var bw = this.transitionProb(trace, oldTrace, this.revProposalDist);
-    var p = Math.exp(ad.value(trace.score) - ad.value(oldTrace.score) + bw - fw);
+
+    var newTraceScore, oldTraceScore;
+    if (this.factorCoeff == 1) {
+      // Optimise for the common case.
+      newTraceScore = ad.value(trace.score);
+      oldTraceScore = ad.value(oldTrace.score);
+    } else {
+      newTraceScore = ad.value(trace.scoreAllChoices()) + this.factorCoeff * ad.value(trace.scoreAllFactors());
+      oldTraceScore = ad.value(oldTrace.scoreAllChoices()) + this.factorCoeff * ad.value(oldTrace.scoreAllFactors());
+    }
+
+    var p = Math.exp(newTraceScore - oldTraceScore + bw - fw);
     assert(!isNaN(p));
     return Math.min(1, p);
   };
diff --git a/src/trace.js b/src/trace.js
index 46432a25..cfe2728c 100644
--- a/src/trace.js
+++ b/src/trace.js
@@ -59,6 +59,8 @@ Trace.prototype.addChoice = function(dist, val, address, store, continuation, op
   // assert(_.isObject(store));
   // assert(_.isFunction(continuation));
 
+  var choiceScore = dist.score(val);
+
   var choice = {
     k: continuation,
     address: address,
@@ -67,6 +69,7 @@ Trace.prototype.addChoice = function(dist, val, address, store, continuation, op
     // Record the score without adding the choiceScore. This is the score we'll
     // need if we regen from this choice.
     score: this.score,
+    choiceScore: choiceScore,
     val: val,
     store: _.clone(store),
     numFactors: this.numFactors
@@ -75,10 +78,20 @@ Trace.prototype.addChoice = function(dist, val, address, store, continuation, op
   this.choices.push(choice);
   this.addressMap[address] = choice;
   this.length += 1;
-  this.score = ad.scalar.add(this.score, dist.score(val));
+  this.score = ad.scalar.add(this.score, choiceScore);
   // this.checkConsistency();
 };
 
+Trace.prototype.scoreAllChoices = function() {
+  return this.choices.reduce(function(acc, choice) {
+    return ad.scalar.add(acc, choice.choiceScore);
+  }, 0);
+};
+
+Trace.prototype.scoreAllFactors = function() {
+  return ad.scalar.sub(this.score, this.scoreAllChoices());
+};
+
 Trace.prototype.complete = function(value) {
   // Called at coroutine exit.
   assert.strictEqual(this.value, undefined);
diff --git a/tests/test-data/stochastic/expected/partition1.json b/tests/test-data/stochastic/expected/partition1.json
new file mode 100644
index 00000000..977fcfcc
--- /dev/null
+++ b/tests/test-data/stochastic/expected/partition1.json
@@ -0,0 +1,3 @@
+{
+  "logZ": 0
+}
diff --git a/tests/test-data/stochastic/expected/partition2.json b/tests/test-data/stochastic/expected/partition2.json
new file mode 100644
index 00000000..168fd70c
--- /dev/null
+++ b/tests/test-data/stochastic/expected/partition2.json
@@ -0,0 +1,3 @@
+{
+  "logZ": -0.6539264674066638
+}
diff --git a/tests/test-data/stochastic/expected/partition3.json b/tests/test-data/stochastic/expected/partition3.json
new file mode 100644
index 00000000..4f94f556
--- /dev/null
+++ b/tests/test-data/stochastic/expected/partition3.json
@@ -0,0 +1,3 @@
+{
+  "logZ": -2.266
+}
diff --git a/tests/test-data/stochastic/models/partition1.wppl b/tests/test-data/stochastic/models/partition1.wppl
new file mode 100644
index 00000000..caa72c26
--- /dev/null
+++ b/tests/test-data/stochastic/models/partition1.wppl
@@ -0,0 +1,3 @@
+var model = function() {
+  return flip();
+};
diff --git a/tests/test-data/stochastic/models/partition2.wppl b/tests/test-data/stochastic/models/partition2.wppl
new file mode 100644
index 00000000..25a66360
--- /dev/null
+++ b/tests/test-data/stochastic/models/partition2.wppl
@@ -0,0 +1,6 @@
+var model = function() {
+  var x = flip(0.2);
+  var y = flip(0.8);
+  observe(Bernoulli({p : x && y ? 0.1 : 0.6}), true);
+  return {x, y};
+};
diff --git a/tests/test-data/stochastic/models/partition3.wppl b/tests/test-data/stochastic/models/partition3.wppl
new file mode 100644
index 00000000..10612bb0
--- /dev/null
+++ b/tests/test-data/stochastic/models/partition3.wppl
@@ -0,0 +1,7 @@
+// The exact normalization constant for this model is:
+// exp(-1) / (2 * sqrt(pi))
+var model = function() {
+  var mu = gaussian(0, 1);
+  observe(Gaussian({mu, sigma: 1}), 2);
+  return mu;
+};
diff --git a/tests/test-inference.js b/tests/test-inference.js
index dc61305d..4f6fe8b5 100644
--- a/tests/test-inference.js
+++ b/tests/test-inference.js
@@ -754,6 +754,21 @@ var tests = [
       dream2: true,
       dream3: true
     }
+  },
+  {
+    name: 'AIS',
+    settings: {
+      args: {
+        steps: 500,
+        samples: 100
+      },
+      logZ: {check: true, tol: 0.05}
+    },
+    models: {
+      partition1: {logZ: {check: true, tol: 1e-6}},
+      partition2: true,
+      partition3: true
+    }
   }
 ];